Build Your Own Combat Robot

Chapter 7: Controlling Your Motors 135
In Figure 7-6, we assume two low-current, SPDT control switches to drive the
relay coils, although smaller relays with Type C (or SPDT) contacts can be used in
place of the switches. Note that just like the relays described in the previous section,
the control switches have NO, NC, and common terminals. In the resting state,
the NC legs on the forward and reverse switches result in relays A and C being energized
and relays B and D being de-energized. No battery current can flow
through the motor because no path exists from the motor to the negative terminal
of the battery, and the motor terminals are shorted to each other through relays A
and C. The motor is stationary and locked in place.
To run the motor forward, the forward switch is activated, which causes relay
C to de-energize and relay D becomes energized. The motor now has one terminal
connected to the positive side of the battery through relay A, and the other terminal
connected to the negative side of the battery through relay D. This makes a
complete circuit and causes the motor to run. To run the motor in reverse, the reverse
switch is activated, causing a current flow from the battery, through relay C
into the motor and out through relay B into the other side of the motor.
note If both the forward-going and reverse-going switches are activated, the circuit path
will be broken and the motor terminals will be shorted together.
A significant danger of relay control is the possibility of contacts bouncing on
severe impact that a combat robot will receive during a battle. A severe shock impact
in a direction relative to the relay orientation can be sufficient to overcome
the force of the return spring holding the contact bar out, thus causing a momentary
connection across the relay’s contacts. Having a weapon motor switch on for
a moment might not be a catastrophic event, but it can be dangerous if people are
nearby and a weapon starts to move. If a momentary short occurs within the motor
braking relay while the motor is running, or if one of the nonactive relays in the
H-bridge is shorted while the other side of the H-bridge is active, a dead short
across the main motor batteries will result. In the relay circuit shown Figure 7-6,
this can happen even when the motor is not running—because half the relays in
the circuit are always energized, a momentary contact bounce of any of the
non-energized relays will cause a catastrophic short. The dead-short battery current
will inevitably weld the contacts together, resulting in the entire wiring harness
going up in smoke and one dead robot.
Turning Switches On and Off
In a remote controlled robot, you will need a way to turn switches on and off remotely.
This can be done either electronically or mechanically. The electronic approach
will be discussed in the solid-state logic section. A mechanical approach
will require some form of an actuator to turn the switch on and off physically. One
of the cheapest and easiest ways to mechanically actuate a switch is to simply use a
standard hobby radio-controlled (R/C) servo to throw a switch.